This project is concerned with investigating basic neural mechanisms in hearing via the techniques of single cell recording in monkeys. Both natural sounds (tape recorded monkeys vocalizations) and artificial stimuli are employed. The work undertaken during FY83 has focused on the nature and extent of two-tone inhibition in the auditory cortex. The phenomenon of two-tone inhibition has been well studied at lower centers of the auditory system, and is undoubtedly an expression of widespread processes for boundary sharpening in sensory systems. Our studies are aimed at providing a description of this process as reflected at the highest auditory center, and at identifying cells whose network properties appear to make them likely candidates for the role of complex feature detectors. Approximately 75 cells have been isolated in primary auditory cortex of awake squirrel monkeys and tested with computer driven series of single and double or combined tones. A wide range of intensities are included as well as differing degrees of temporal overlap between the two tones tested. As found in our laboratory earlier a substantial number of cells (about 25%) have poorly defined tuning curves. These cells have in general high spontaneous rates of discharge and respond with a reduction in rate to tones (inhibitory response). The remainder have definable Best Frequencies and demonstrate some degree of two-tone inhibition. A classification of these cells is being developed.